Bengal Delta, Africa Rifting, India Sand Mining

A few readings for your perusal- 

1) The future of Bengal Delta. With this succinct title Dipen Bhattacharya has written an informative article on the origin and evolution of the Bengal Delta. The Bay of Bengal was created when India broke away from eastern Antarctica about 130 -120 million years ago in the early mid Cretaceous. The basin expanded as India drifted northwards. From Cretaceous to Oligocene times (120-25 million years ago) rivers from Peninsular India were providing most of the sediment being deposited in the Bay. Himalaya derived sediment started overwhelming Peninsular river input from about 25 million years ago. K.S. Krishna and coworkers have very elegantly demonstrated this in their study of sediment pathways in to the Bay of Bengal.

Dr. Bhattacharya has traced the evolution of the delta into more recent times, explaining the role of the Pleistocene ice ages in delta growth. The delta’s future too is at risk with dam building and ground water extraction amplifying the changes due to global warming induced sea level rise. Well worth reading.

2) Eastern Africa Is Splitting Apart, but Not Where We Expected.  Africa is tearing apart along a north south oriented corridor from the Red Sea to Mozambique. Plate motion has formed the famous rift valleys of Ethiopia, Kenya, and Tanzania, as the crust stretches and subsides along faults. Kimberley Cartier explains the geological set up of the region and the stages in which continents break apart with oceanic basins eventually forming along the initial zones of continental rifts.

Why this region of Africa is rifting is not all that easy to explain. If you look at the plate tectonic map of Eastern Africa and the adjacent Indian Ocean and Arabian Sea you will notice that the oceanic Somalia and Indian Ocean tectonic plates are pushing into East Africa. For continents to split and be pulled apart, there have to be extensional forces generated. These are usually provided at the locus of rifting by the mantle doming up, thereby breaking and pushing the lithosphere away, and by one end of the plate subducting underneath another overriding plate. The subducting oceanic crust becomes denser and heavier as it sinks deeper, pulling the rest of the plate with it.

After the breakup of Gondwanaland, the northerly movement of the India plate through the Cretaceous was sustained by the pull force of the northern edge of the India plate sinking under Asia. On the other hand, Eastern Africa is surrounded by plate spreading zones. There is no pull force available for eastern Africa, only the localized extensional stresses due to mantle upwelling.  Is that providing adequate horizontal traction at the base of the Africa Plate for the crust to break apart and stretch? For a deeper understanding into the mantle forces responsible for this, I will recommend J Micheal Kendall and Carolina Lithgow-Bertelloni ‘s article- Why is Africa Rifting?

3)  India’s rivers bear lasting scars from relentless sand mining. Some years ago I heard a podcast on Planet Money about a Jamaican beach that was stolen. An estimated 500 truckloads of sand was hauled away in the middle of the night. Sand is big business all over the world. Indian river beds too are being plundered for their sand to satisfy the demands of the booming construction industry. Sahana Ghosh explores how scientists are surveying Indian rivers using field observations and satellite data. They are trying to track down the amount of sand being extracted and the environmental impact of sand mining.

Human Impact On Earth's Sediment Cycle

One common type of argument I hear from anthropogenic climate change deniers is that human activity is too insignificant to affect the balance of global natural processes. On one debate a participant claimed that one large volcanic eruption emits more carbon dioxide than that by human activity. The actual amounts contradict this claim. Volcanism on earth emits about 0.13 -0.44 billion tons of CO2 per year. Human activity on the other hand emits about 35-40 billion tons of CO2 per year.

Jaia Syvitski and colleagues have produced a similar eye opening review of the human impact on earth's sediment cycle. The production, mobilization , transport, and deposition of sediment is based on a balance between tectonic processes, climate, erosion, and human activities. Our impact on sediment movement and its sequestration has now become so large that it dwarfs natural processes. 

The paper is open access for a limited time. Earth's sediment cycle during the Anthropocene. 

It is dense reading, full of numbers on sediment loads and fluxes.

"Human activities have increased fluvial sediment delivery by 215% while simultaneously decreasing the amount of fluvial sediment that reaches the ocean by 49%, and societal consumption of sediment over the same period has increased by more than 2,500%".

or: The Indus River once transported about 270 million tons of sediment to its delta. It presently deposits only about 13 million tons per year. So much of Indus water is siphoned off by canals, that it  often turns dry before reaching the sea.  

 and one more: "Large dams have trapped about 3,200 Gt of sediment since 1950 (ref.123), approximately 74% of which would likely have reached the coastal ocean". (Gt =billion tons)

There are many such stories from around the globe about the staggering amounts of sediment extracted and redirected for human use. Next time, don't shrug off the news you read about unregulated sand mining from our rivers. It is causing serious damage to riverine and coastal ecosystems.

The review ends with a proposal to set up a ‘Earth Sediment Cycle Grand Challenge’, a collaborative effort to better understand the changes to the sediment cycle. Such an initiative we surely need to address the many ongoing and future threats to our rivers and deltas.

Articles: Dehradun, Early Dogs, Warm Blooded Dinos, Louisiana Delta

Sharing some links from the past few days:

1) Dehradun.

A story of the transformation of a beautiful hill town to an ugly unplanned urban center. We shrug resignedly at many such tales from across the country. This one is of Dehradun. Himalaya towns can only be described as disasters in the making. Unscientifically built infrastructure on steep slopes, no garbage management resulting in enormous stray dog and pig populations roaming the streets, and a dwindling water supply. Yet these towns continue to grow, pointing to worsening opportunities for making a livelihood in the Himalayan rural landscapes. The 'smart city' reference is the ultimate insult of all.

Vanishing landscape of ‘smart city’ Dehradun.

2) Early Dogs.

The early stages of dog domestication may have seen a marked behavioral shift appearing before any distinct morphological change. This change in behavior, arising from docile wolves or 'protodogs' living near human camps would have entailed a change in diets.  Scientists have compared wolf and dog like remains from a 28,500 year old site in the Czech Republic. They looked at the dental microwear pattern of these two groups of canids and noticed that the dog like canids show a pattern consistent with eating more hard brittle foods. The wolves show patterns consistent with eating more flesh. 'Throw this dog a bone" wasn't an insult then.

Dental microwear as a behavioral proxy for distinguishing between canids at the Upper Paleolithic (Gravettian) site of Předmostí, Czech Republic

write up: Dog domestication during ice age.

3) Warm Blooded Dinos?

Were Dinosaurs warm blooded like mammals? This debate has raged on for decades. Bone growth patterns have not given any unambiguous evidence of body temperature regulation. A new method known as clumped isotopes may provided a more reliable indicator of estimating body temperatures. Fossilized dinosaur egg shells contain the original calcium carbonate from which these shells were built. A variety of carbon isotopes (C12, C13) may bond with a variety of oxygen isotopes (O16, O17, O18) in the carbonate molecules (CO3). The degree of bonding or clumping of the heavier isotopes i.e. C13 to O18 varies with the temperature during mineral growth. Clumping is more at lower temperatures.

Scientists compared this C13-O18 clumpiness in dinosaur egg shells with C13-O18 clumpiness in the calcium carbonate of mollusc shells from the same fossil bed. Mollusc geochemistry is taken to be a proxy for the ambient conditions. They found out that the egg shells grew at temperatures between 25- 43 deg C, while the molluscs record growth at 25 -30 deg C. This suggests that dinosaurs were capable of maintaining a higher body temperatures than their surroundings.  As a carbonate sedimentologist, I found the details of methods in this paper  of great interest. The researchers used a variety of techniques to make sure that the egg shells had not been altered or subjected to higher temperatures later in their history, which would have made them an unreliable archive of the original temperature during growth. The analyzed egg shells came from Sauropods, Theropods and Ornithischians, a sample across the three main groups of dinosaurs. Very interesting study.

Eggshell geochemistry reveals ancestral metabolic thermoregulation in Dinosauria

write up - Fossil Eggshells Suggest All Dinosaurs May Have Been Warm-Blooded

4)  Eroding Louisiana Coastline.

Over the past several decades, barrages and levees have drastically reduced the amount of sediment that the Mississippi river is carrying to the sea. As a result, the famed Mississippi delta and coastline is eroding away. Efforts are on in a Boston warehouse to figure out a way to reverse this change. An ambitious engineering project which aims at opening up a portion of the levee to funnel sediment into the Barataria Basin south of  New Orleans is being planned. The hope is that the new channel will transport and deposit enough sediment to rebuild part of the endangered delta. A scale model built in a warehouse near Boston is testing the efficacy of this idea.

Fascinating to read the various problems geologists and engineers have to deal with when grappling with modifying nature at this scale.

To Save Louisiana’s Vanishing Coast, Build a Mini Mississippi Near Boston.

W. Bengal Bangladesh- Geologic Controls On Arsenic Distribution In Ground Water

Science writer  Priyanka Pulla has written an excellent article exploring the geologic, socio-economic and technological issues related to the widespread arsenic contamination of groundwater in W. Bengal. Sadly, the government response to this crisis has been slow.

I thought I would elaborate on the geological question -  Why are Arsenic (As) levels much higher in shallower Holocene age aquifers and lower in the deeper Pleistocene age aquifer? The answer encompasses mineralogy, climate change, sea level changes and bacteria.

The ultimate source of As are high Himalayan rocks and Indo-Burman ranges with additional contributions from the Precambrian terrains of Peninsular India and the Siwalik hills.  Minerals like biotite, magnetite, illmenite, olivine, pyroxene, amphiboles contain As. These minerals release As when they undergo weathering in catchment areas and deposits of the alluvial plains. This As is absorbed on secondary minerals like Fe hydroxides like goethite. Such Fe hydroxides are authigenic, i.e. they grow in the shallow buried sediments of the alluvial plains. Under oxidizing conditions, As is immobile, sequestered in Fe hydroxides. However,  conditions may change, and these sediments may get overlain by or be redeposited in environments rich in organic material. Certain bacterial species living on this organic material break down these Fe hydroxides, using the oxygen for their metabolism, and releasing Fe and As into the groundwater. This is known as reductive dissolution of Fe hydroxides and is the principal mechanism for As entering the groundwater in the alluvial plains of Bangladesh and West Bengal.

During the Pleistocene.. 1) the high Himalaya was glaciated. Therefore, important sources of As like the Fe-Mg rich rocks of the Indus ophiolite belt (slices of oceanic crust that existed between India and Asia which have been thrust up during continental collision) and high grade metamorphic rocks such as schists and gneisses were covered in ice and not releasing sediment. Indian cratonic areas, the Siwalik foothills  and the Indo-Burman ranges were being eroded, but overall less As was making its way on to alluvial plains. 2) Since climate was cooler and drier, there was less organic material accumulating in sediment of alluvial plains. Conditions were oxidizing and As remained sequestered in Fe hydroxide minerals. 3) Sea level was much lower then. Almost the entire continental shelf was dry land. Ganga and Brahmaputra met the sea much to the south of present shoreline. Reducing environments like delta front marshes, ponds, estuaries, existed much to the south.

Sedimentary conditions changed by 12-15 thousand years ago. Glacial melt exposed As bearing rocks in high Himalaya. As a result, more As made its way on to alluvial plains. Importantly, sea level rose and flooded the continental shelf. The Pleistocene delta front reducing environments were drowned. Shorelines shifted northwards. The climate was warmer, encouraging vegetation growth. Reducing delta front environments like swamps, coastal marshes and lakes developed on previous alluvial plain sediments.

The map below shows the position of shorelines between 7 thousand and 4 thousand years ago along with the location of wells with high levels of As. This study focuses on Bangladesh but similar conditions existed in West Bengal as well. The sea has receded 2- 3 meters to its present location since 4 thousand years ago.  The delta front and shoreline belt that existed 4-7 thousand years ago is now a densely inhabited region .

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

Notice clustering of wells with high As along the past shorelines. Here, organic rich delta marshes and swamps developed. Bacterial reduction of Fe hydroxides released As in to groundwater.

As distribution also shows correlation with topography. This map shows high As levels in groundwater coinciding with topographic lows. Such low lying areas accumulate more fine sediment and organic material. Again, this will apply also to W. Bengal.

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

So, a change in climate and shifts in sedimentary environments in response to changing sea level from Pleistocene to Holocene exerted a strong control on As distribution in the alluvial plains of Bangladesh and W. Bengal. 

The Bay Of Bengal Once Touched Sikkim

See this satellite imagery of the Himalaya.  The Indian State of Sikkim occupies the region just east of Darjeeling.

The Siwaliks (green arrows) appear as a forested linear band forming the southernmost hilly terrain of the Himalaya. The hills abut against broad alluvial plains. Rivers traversing the Himalaya carrying enormous sediment load encounter a gentler gradient upon exiting the hilly terrain. A loss of stream power results in sediment being dumped in the channel, so much so, that rivers get chocked on their own sediment. As a result, channels split and bifurcate forming a braided river system. These rivers  also suddenly change course, abandoning their channel and carving out new ones. Such course changes may occur during floods or by tilting of the land by structural movements.  Over time, the deposits of these ever changing rivers coalesce to form cone shape aprons of sediments known as alluvial fans. These rivers like the Kosi and the Tista, which flow transverse to the mountain range, meet an axial river like the Ganga and the Brahmaputra flowing parallel to the mountain front. The axial river flows into the Bay of Bengal.

The Siwalik hills were once these type of alluvial fans.  Just as today, during Miocene and Pliocene times, sediment was being deposited in front of the rising Himalayan mountains. Beginning about half a million years ago or so, these ancient alluvial fans were crumpled up and uplifted to form the Siwalik ranges. Active alluvial fan formation shifted southwards to its present locus. This process continues. In a few million years, the present day alluvial fans deposited by rivers like the Kosi and the Teesta will be deformed into a newer mountain range south of the Siwaliks. The Himalaya are growing southwards.

How do we know that the Siwaliks were once alluvial fans? Geologists rely on analogy, comparing the Siwalik sediments with what is accumulating in the present day alluvial fans. They find a striking similarity. Siwaliks are made up of alternations of coarse gravel layers and finer sand and silt layers with characteristic bed orientations and structures like cross beds and rippled sand. The gravel layers are inferred to be the river channel deposits while the finer sand and silt layers are the river bank, levee and floodplain deposits. An important finding made throughout the length of the Siwalik ranges has been the paleo-current directions preserved in the rocks.  Geologists have measured the orientation of bedding and ripple marks and found out that rivers were flowing south and south east i.e. perpendicular to the mountain chain. There is no evidence of an axial river like the Ganga in these Siwalik sediments. The thinking is that such an axial river must have flowed much to the south of the region of deposition of Siwalik sediments.

And what about evidence of a delta? Where did these Miocene and Pliocene rivers meet the sea? The logical geographic place to look for a coast would be towards the east. And in fact, that evidence has come from the Siwalik sediments of West Bengal and Sikkim. In a really interesting paper published recently in Current Science, Suchana Taral, Nandini Kar and Tapan Chakraborty describe sedimentary structures and marine trace fossils from Middle Siwalik sediments exposed along the Gish River and its tributaries in the Tista Valley. Siwalik rocks in the central and western part of the Himalaya show current structures that indicate south flowing rivers. In this easterly location however, the sediments show evidence of being deposited in a wave influenced environment. Sedimentary structures like wave ripple laminations and hummocky-swaley stratification indicate deposition in wave dominated marine bay.  Paleo-current indicators like ripple marks preserved on sandstone surfaces show a south as well as north directed current. This suggests an environment influenced by tides and north directed waves. Associated sediments show indicators of different delta environments like distributary channels, delta mouth bar and delta flood plain deposits.

Apart from current direction indicators, the sediments contain plant fossils indicative of mangrove vegetation and brackish water environments. They also contain trace fossils i.e. impressions and burrows made by creatures moving and disturbing the sediment surface. Cylindrichnus, Chondrites, Rosselia, Taenidium, Skolithos, Planolites are some of trace fossils reported in this study. The assemblage of trace fossils is similar to those reported from marine settings.

All this suggests that during the time of deposition of these Middle Siwalik sediments in Late Miocene-Pliocene times, about 5-10 million years ago, a branch of the Bay of Bengal had invaded as far north as present day Sikkim. Rivers carrying sediment from the Himalaya were debouching them in a delta and a shallow marine bay. The Sikkim Middle Siwalik strata are ancient deformed delta and marine deposits.  

A paleo-geographic reconstruction of this eastern part of these Siwalik depositional environments in shown below.

 Source: Suchana Taral, Nandini Kar and Tapan Chakraborty 2017

The  upper graphic shows the reconstructed delta and marine depositional environment. The lower graphic shows the regional paleo-geography. The pin shows the environmental location of the study area. The yellow rose diagram shows the paleocurrent directions measured in the Siwalik sediments.

Interestingly, some earlier work by geologists has shown that in Late Miocene times the Brahmaputra was flowing along a much more easterly route towards the Bay of Bengal. They used sand thickness and sand/shale ratios from wells drilled in the delta and found lobate sand bodies, which they inferred were brought in by a large river flowing from a ENE source. Their interpretation is shown in the graphic to the left (Uddin A. and Lundberg N. 1998). At the time the Shillong Plateau did not exist. The river flowed into the Bay of Bengal from the Upper Assam valley and through the Sylhet depression in to the Bengal Basin. The uplift of the Shillong Plateau in Pleistocene times forced the Brahmaputra to turn west and wrap itself around the newly emerging uplands.

Since Pliocene times, the tremendous amount of sediment being delivered by Himalayan rivers, coupled with Pleistocene sea level fall, has caused a retreat of this arm of the Bay of Bengal southwards.

In the satellite image below, based on the location of the Sikkim Siwalik deposits and other work on the Bengal Basin paleogeography, I have drawn in brown the coastline as it would have existed 5-10 million years ago. The ancient drainage systems are shown in blue. South directed arrows shows the extent of the growth of the Bengal/Bangladesh alluvial plains and delta and the retreat of the sea since then to its present location.

Pretty amazing finding.

Sedimentation Patterns Bay Of Bengal: How Old Is The River Ganga

...In our legends it is said that the goddess Ganga's descent from the heavens would have split the earth had Lord Shiva not tamed here torrent by tying it into his ash-smeared locks. To hear this story is to see the river in a certain way: as a heavenly braid, for instance, an immense rope of water, unfurling through a wide and thirsty plain. That there is a further twist to the tale becomes apparent only in the final stages of the river's journey - and this part of the story always comes as a surprise, because it is never told and thus never imagined. It is this : there is a point at which the braid comes undone; where Lord Shiva's matted hair is washed apart into a vast knotted tangle. Once past that point the river throws off its bindings and separates into hundreds, maybe thousands of tangled strands....

Amitav Ghosh- The Hungry Tide

A study by K.S Krishna and colleagues published recently in Current Science shows very elegantly using sesimic reflection profiles and sediment isopach maps how the Bay of Bengal has been filling up with sediment since Late Cretaceous times.

The Bay of Bengal (BoB) originated with the rifting of India from Antarctica by early Cretaceous, thus forming the Indian east coast margin. The depression over time evolved into an ocean basin with new oceanic lithosphere forming in the Bay of Bengal at sea floor spreading centres. Its conjugate oceanic crust is probably beneath the Enderby Basin at the margin of East Antarctica. In this depression, an enormous volume of sediment has been deposited from mid-late Cretaceous to recent times. The age ranges of sediment packages, their distribution, geometry and thickness tell us about the changing source regions of these sediments and the influence of the rising Himalayas and the monsoons on sedimentation history.

Scientists involved in this study used seismic  reflection profiles to construct a seismic stratigraphy of the sediment pile in the BoB. The ages ranges of the sesimic sequence were then calibrated using biostratigraphy erected from two deep sediment cores from the vicinity of the seismic lines. Thus, a Cretaceous to recent subdivision of the mega sequence into distinct depositional episodes separated by unconformities could be recognized. The seismic profiles also revealed the geometry and thicknesses of the sedimentary sequences and the topography of the basement.

Several coast perpendicular grabens (linear depressions) were clearly outlined. These are continuations of ancient Archaean and Proterozoic sutures zones and rifts now occupied by the major rivers of Peninsular India, the Cauvery, Krishna, Godavari and the Mahanadi. Sedimentary packages until the late Oligocene are thicker near the east coast and thin out into the deeper shelf areas. From early Miocene onwards the sediment packages are thicker near the Ganges Brahmaputra delta and in the deeper shelf areas to the east and north and thin towards the shallower coastal shelf. This implies a changes in direction of sediment delivery systems, with the Indian craton being the major source in the earlier phase, to the Himalayas and the Indo Burman ranges being the major source in the younger phase.

This is brought out beautifully by sediment isopach maps constructed for several time slices. Isopach maps show the thickness of sediments for a particular time slice. In this case four time slices were used: 1) Basement to Late Cretaceous 2) Late Cretaceous to Oligocene 3) Oligocene top to Late Miocene 4) Late Pleisocene to recent.

The results are shown below and they clearly show changing sediment sources and distribution pathways.

From Cretaceous to the Oligocene, thick sediment wedges coincide with the Peninsular river grabens indicating that sediments derived by erosion of the Indian craton was the major source to the BoB.

 Source: K.S Krishna et al. 2016

From late Oligocene throughout much of the Miocene the sediments are thickest in the Ganges Brahmaputra delta region and thin out southwestwards towards the east coast shelf area.

 Source: K.S Krishna et al. 2016

This indicates that river systems eroding the rising Himalayas were now the major suppliers of sediment to the BoB. Sediment thickness from late Miocene to mid Pleistocene also show enhanced sedimentation from the north and this pattern coincides with an increase in the Asian monsoon. From Pleistocene to recent times, there has been more sediment from the Godavari Krishna system to the Bob, while a strong sediment delivery system from the Ganges-Brahmaputra continues.

This is the end of part 1 of the post, but I had an intriguing question..

How old is the river Ganga of the plains, flowing from the Himalayan front near Haridwar to the Bay of Bengal?  What does the data from the Bengal Basin tell us about paleo-rivers and how do geologists go about collecting and analyzing this data?

The Ganges Delta And The Hungry Tide

Amitav Ghosh is his book The Hungry Tide  evocatively describes the Ganges -Brahmaputra delta, the place where these mighty rivers change form as in from a one major active meandering channel into many entangled entities:

In our legends it is said that the goddess Ganga's descent from the heavens would have split the earth had Lord Shiva not tamed here torrent by tying it into his ash-smeared locks. To hear this story is to see the river in a certain way: as a heavenly braid, for instance, an immense rope of water, unfurling through a wide and thirsty plain. That there is a further twist to the tale becomes apparent only in the final stages of the river's journey - and this part of the story always comes as a surprise, because it is never told and thus never imagined. It is this : there is a point at which the braid comes undone; where Lord Shiva's matted hair is washed apart into a vast  knotted tangle. Once past that point the river throws off its bindings and separates into hundreds, maybe thousands of tangled strands.

Until you behold it for yourself, it is almost impossible to believe that here, interposed between the sea and the plains of Bengal, lies an immense archipelago, stretching for almost three hundred kilometers, from the Hoogly River in West Bengal to the shores of the Meghna in Bangladesh. 

The islands are the trailing threads of India's fabric, the ragged fringe of her sari, the achol that follows her, half-wetted by the sea. They number in the thousands, these islands; some are immense and some no larger than sandbars; some have lasted through recorded history while others were washed into being just a year or two ago. These islands are the rivers restitution, the offerings through which they return to the earth what they have taken from it, but in such a form as to assert their permanent dominion over their gift. The rivers channels are spread across the land like a fine mesh net, creating a terrain where the boundaries between land and water are always mutating, always unpredictable.

The delta and its front end - the sediment fan -  that is present below sea level in the Bay of Bengal is essentially made up of Himalayan sediment worn down by the many rivers and redeposited in the Bay. A pile of sediment more than 10 km thick have accumulated since the early Cenozoic.  Image below shows the Ganges Brahmaputra delta and the region called the Sunderbans named for the mangrove forests that grow on the thousands of sandbars and islands.